This invention relates to an open architecture superconducting magnet assembly for a magnetic resonance imager (hereinafter called "MRI").
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of resistance, thereby maintaining a magnetic field. Superconducting magnets find wide application in the field of MRI.
Considerable research and development efforts have been directed at eliminating the need for a boiling cryogen such as liquid helium. While the use of liquid helium to provide cryogenic temperatures has been widely practiced and is satisfactory for MRI operation, helium is found and commercially obtained only in the state of Texas. As a result, the provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly. This has led to considerable effort being directed at superconducting materials and magnet structures which can be rendered superconducting at relatively higher temperatures such as ten degrees Kelvin (10K), which can be obtained with conduction cooling.
Another problem encountered by most MRI equipments is that they utilize solenoidal magnets enclosed in cylindrical structures with a central bore opening for patient access. However, in such an arrangement, the patient is practically enclosed in the warm bore, which can induce claustrophobia in some patients. The desirability of an open architecture in which the patient is not essentially totally enclosed has long been recognized. Unfortunately, an open architecture structure poses a number of technical problems and challenges. One problem is to provide a suitable coil support and structure which will provide the required magnetic field yet occupies much less space than conventional cylindrical structures, and yet which nevertheless can support the magnet coils under the considerable electromagnetic forces and thermal forces encountered during cool-down from ambient temperature to superconducting temperatures. Moreover, such an arrangement must still be capable of generating the very uniform yet strong magnetic field required. An open architecture MRI with which the subject invention could be used is disclosed in U.S. patent application Ser. No. 07/709,528, filed Jun. 3, 1991 by E. T. Laskar, entitled "Open MRI Magnet", assigned to the same assignee as the subject invention, and hereby incorporated by reference. Also see the patents referenced in that patent application.
Moreover, there are a number of other problems, including problems of differential thermal expansion and contraction of materials, of minimizing cost, and of handling the forces generated by the significant magnetic fields required. All of these overlapping and conflicting requirements must be satisfied for a practical and satisfactory MRI superconducting magnet structure.